Validation of sensor calibration

There is provided a method of validating a calibration of a sensor represented by a calibrated relationship between concentration of an analyte in a sample and measurements from a sensor of an optical property of a sensing substance, wherein the optical property of the sensing substance has a spectrum that varies with the concentration of the analyte in the sample, and the spectrum has an isosbestic wavelength at which the optical property does not vary with concentration of the analyte, the method comprising making measurements of the optical property at three or more wavelengths of light while the sensing substance is exposed to the sample, determining whether the measurements of the optical property are inconsistent with the calibrated relationship, and outputting a warning signal in response to the measurements of the optical property being inconsistent with the calibrated relationship.